1. Field of the Invention
This invention generally relates to improved methods of fabricating molecular electronic devices, in particular organic electronic devices such as organic light emitting diodes (OLEDs) by droplet deposition techniques such as ink jet printing. The invention also relates to molecular device substrates fabricated by and/or use in such methods.
2. Related Technology
Organic light emitting diodes (OLEDs) are a particularly advantageous form of electro-optic display. They are bright, colourful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using either polymers or small molecules in a range of colours (or in multi-coloured displays), depending upon the materials used. A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole injecting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image.
FIG. 1 shows a vertical cross-section through an example of an OLED device 100. In an active matrix display part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 1). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic, on which an anode layer 106 has been deposited. The anode layer typically comprises around 150 nm thickness of ITO (indium tin oxide), over which is provided a metal contact layer, typically around 500 nm of aluminium, sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal may be purchased from Corning, USA. The contact metal (and optionally the ITO) is patterned as desired, and so that it does not obscure the display, by a conventional process of photolithography followed by etching.
A substantially transparent hole injection layer 108a is provided over the anode metal, followed by an electroluminescent layer 108b. Banks 112 may be formed on the substrate, for example from positive or negative photoresist material, to define wells 114 into which these active organic layers may be selectively deposited, for example by a droplet deposition or inkjet printing technique. The wells thus define light emitting areas or pixels of the display.
A cathode layer 110 is then applied by, say, physical vapour deposition. A cathode layer typically comprises a low work function metal such as calcium or barium covered with a thicker, capping layer of aluminium and optionally including an additional layer immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may achieved through the use of cathode separators (element 302 of FIG. 3b). Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress.
Organic LEDs of this general type may be fabricated using a range of materials including polymers, dendrimers, and so-called small molecules, to emit over a range of wavelengths at varying drive voltages and efficiencies. Examples of polymer-based OLED materials are described in WO90/13148, WO95/06400 and WO99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of small molecule OLED materials are described in U.S. Pat. No. 4,539,507. The aforementioned polymers, dendrimers and small molecules emit light by radiative decay of singlet excitons (fluorescence). However, up to 75% of excitons are triplet excitons which normally undergo non-radiative decay. Electroluminescence by radiative decay of triplet excitons (phosphorescence) is disclosed in, for example, “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest Applied Physics Letters, Vol. 75(1) pp. 4-6, Jul. 5, 1999. In the case of a polymer-based OLED layers 108 typically comprise a hole injection layer 108a and a light emitting polymer (LEP) electroluminescent layer 108b. A further hole transport layer (not shown) may be provided between hole injection layer 108a and electroluminescent layer 108b. The electroluminescent layer may comprise, for example, around 70 nm (dry) thickness of PPV (poly(p-phenylenevinylene)) and the hole injection layer, which helps match the hole energy levels of the anode layer and of the electroluminescent layer, may comprise, for example, around 50-200 nm, preferably around 150 nm (dry) thickness of PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene).
FIG. 2, which is taken from WO2005/076386 (hereby incorporated by reference), shows a view from above (that is, not through the substrate) of a portion of a three-colour active matrix pixellated OLED display 200 after deposition of one of the active colour layers. The figure shows an array of banks 112 and wells 114 defining pixels of the display. The wells are formed as apertures in a continuous layer or sheet.
FIG. 3a shows a view from above of a substrate 300 for inkjet printing a passive matrix OLED display. FIG. 3b shows a cross-section through the substrate of FIG. 3a along line Y-Y′.
Referring to FIGS. 3a and 3b, the substrate is provided with a plurality of cathode undercut separators 302 to separate adjacent cathode lines (which will be deposited in regions 304). A plurality of wells 308 is defined by banks 310, constructed around the perimeter of each well 308 and leaving an anode layer 306 exposed at the base of the well. The edges or faces of the banks are tapered onto the surface of the substrate as shown, heretofore at an angle of between 10 and 40 degrees. The banks present a hydrophobic surface in order that they are not wetted by the solution of deposited organic material and thus assist in containing the deposited material within a well. This is achieved by treatment of a bank material such as polyimide with an O2/CF4 plasma as disclosed in EP 0989778. Alternatively, the plasma treatment step may be avoided by use of a fluorinated material such as a fluorinated polyimide as disclosed in WO 03/083960.
As previously mentioned, the bank and separator structures may be formed from resist material, for example using a positive (or negative) resist for the banks and a negative (or positive) resist for the separators; both these resists may be based upon polyimide and spin coated onto the substrate, or a fluorinated or fluorinated-like photoresist may be employed. In the example shown the cathode separators are around 5 μm in height and approximately 20 μm wide. Banks are generally between 20 μm and 100 μm in width and in the example shown have a 4 μm taper at each edge (so that the banks are around 1 μm in height). The pixels of FIG. 3a are approximately 300 μm square but, as described later, the size of a pixel can vary considerably, depending upon the intended application.
Techniques for the deposition of material for organic light emitting diodes (OLEDs) using ink jet printing techniques are described in a number of documents including, for example, Y. Yang, “Review of Recent Progress on Polymer Electroluminescent Devices,” SPIE Photonics West: Optoelectronics '98, Conf. 3279, San Jose, January, 1998; EP 0 880 303; and “Ink-Jet Printing of Polymer Light-Emitting Devices”, Paul C. Duineveld, Margreet M. de Kok, Michael Buechel, Aad H. Sempel, Kees A. H. Mutsaers, Peter van de Weijer, Ivo G. J. Camps, Ton J. M. van den Biggelaar, Jan-Eric J. M. Rubingh and Eliav I. Haskal, Organic Light-Emitting Materials and Devices V, Zakya H. Kafafi, Editor, Proceedings of SPIE Vol. 4464 (2002). Ink jet techniques can be used to deposit materials for both small molecule and polymer LEDs.
A volatile solvent is generally employed to deposit a molecular electronic material (typically an organic semiconducting material), with 0.5% to 4% dissolved solvent material. This can take anything between a few seconds and a few minutes to dry and results in a relatively thin film in comparison with the initial “ink” volume. Often multiple drops are deposited, preferably before drying begins, to provide sufficient thickness of dry material. Solvents which may be used include cyclohexylbenzene and alkylated benzenes, in particular toluene or xylene; others are described in WO 00/59267, WO 01/16251 and WO 02/18513; a solvent comprising a blend of these may also be employed. Precision ink jet printers such as machines from Litrex Corporation of California, USA are used; suitable print heads are available from Xaar of Cambridge, UK and Spectra, Inc. of NH, USA.
Inkjet printing has many advantages for the deposition of materials for molecular electronic devices but there are also some drawbacks associated with the technique. As previously mentioned the photoresist banks defining the wells have until now tapered to form a shallow angle, typically around 15°, with the substrate. However it has been found that dissolved molecular electronic material deposited into a well with shallow edges dries to form a film with a relatively thin edge. FIGS. 4a and 4b illustrate this process.
FIG. 4a shows a simplified cross-section 400 through a well 308 filled with dissolved material 402 (ink), and FIG. 4b shows the same well after the material has dried to form a solid film 404. In this example the bank angle is approximately 15° and the bank height is approximately 1.5 μm. The solution 402 has a contact angle θc with the plasma treated bank material of typically between 30° and 40° for example around 35°; this is the angle the surface of the dissolved material 402 makes with the (bank) material it contacts, for example angle 402a in FIG. 4a. As the solvent evaporates the solution becomes more concentrated and the surface of the solution moves down the tapering face of a bank towards the substrate; pinning of the drying edge can occur at a point between the initially landed wet edge and the foot of the bank (base of the well) on the substrate (“pinning” is the point during evaporation at which the diameter d of the solution 402 remains essentially constant even though the volume V of the solution is decreasing—in other words Δd/ΔV=0, or is very small as compared to the drop diameter). The result, shown in FIG. 4b, is that the film of dry material 404 can be very thin, for example of the order of 10 nm or less, in a region 404a where it meets the face of a bank. In practice drying is complicated by other effects such as the coffee ring—effect. With this effect because the thickness of solution is less at the edge of a drop than in the centre, as the edge dries the concentration of dissolved material there increases. Because the edge tends to be pinned solution then flows from the centre of the drop towards the edge to reduce the concentration gradient. This effect can result in dissolved material tending to be deposited in a ring rather than uniformly. The physics of the interactions of a drying solution with a surface are extremely complicated and a complete theory still awaits development.
A problem with inkjet deposition arises when filling wells which are large compared with the size of an inkjet droplet. One way around this problem is to sufficiently over fill the well such that the dissolved material is pushed into the well corners. This can be achieved by using a large number of dilute droplets and a high barrier around the well. Techniques for depositing large volumes of liquid are described in WO 03/065474 which describes the use of very high barriers (for example at page 8 lines 8 to 20) to allow the wells to hold a large volume of liquid without the liquid overflowing to adjacent wells. However such structures cannot easily be formed by photolithography and instead a plastic substrate is embossed or injection moulded.
US 2005/133802 discloses a bank structure formed by patterning a layer of a photosensitive material, wherein the bank has a structure selected so as to deal with the problem of inaccurate inkjet droplet deposition. The bank structure is designed to contain inkjet drops that are deposited between the wells rather than in the wells; the problems described above are not addressed.
We have previously described some solutions to well-filling issues (WO 05/076386, see in particular pages 6 and 7; and UK patent application no. 0605128.8 filed 15 Mar. 2006 and equivalents thereto incorporated by reference). We now describe some further advantageous techniques addressing this and related problems.